1. Field of the Invention
The invention relates to a method and a device for regulating or controlling the coating or layer thickness, especially the bond coating thickness and can be used in particular in the production of DVDs (digital versatile disks, i.e. versatile writable and readable storage disks).
2. Description of the Related Art
DE-C1-196 05 601 already describes a device for a controlled surface coating. By means of a nozzle which can be moved parallel to a substrate surface, a lacquer or varnish is applied uniformly, and by a digitally controllable step motor, the nozzle can be moved to any desired location during operation, and thus the surface to be coated can be determined. The influence of the temperature of the substrate to be coated, the temperature of the coating material and its viscosity are not taken into account during coating.
DE-A1-38 22 835 discloses a method and a device for lacquering or varnishing workpiece surfaces. During the operating cycle of a robot, a spraying gun obtains from the robot""s control unit a continuously or gradually varying desired value for the lacquer flow. Moreover, the lacquer flow to the spraying gun is measured and readjusted by adjusting the flow resistance on the flow path between lacquer distributor and spraying gun in accordance with its deviation from the present desired value. Moreover, during one operating circle of the robot, the robot""s control unit adjusts continuously or gradually varying values for the sprayer and/or horn air flow of the spraying gun. The method relates to the application of lacquer by means of a spraying gun and thus differs basically from the coating method of the present invention in which the coating material is applied by a dosing pump, a dosing arm being movable over the substrate and a rotary drive for rotating the substrate. In this method, especially the quality of the coating of lacquer is important. In particular, it is intended to avoid the occurrence of drops or blots during the application of lacquer. Therefore, the lacquer flow is adapted to the sprayer air. The problem of regulating the thickness of the coating material is not mentioned in the cited reference.
It was found that there is a reproducible relation between the temperature of the substrates to be coated, the temperature of the coating material and the viscosity of the coating material, on the one hand, and the expected coating thickness during bonding of substrates, on the other hand. FIG. 3 shows, for example, the dependency of the viscosity of the bonding material on the temperature. It was found that if the substrate temperature changes from 40xc2x0 to 45xc2x0 C., the bond coating thickness changes from 40 to 35 xcexcm. For many fields of application, in particular in the case of DVDs, it is of great importance that the bond coating thickness meets narrow tolerances.
Therefore, it is an object of the present invention to provide a method and a device for regulating the coating thickness, wherein a reproducible high accuracy of the coating thickness is achieved.
In achieving the object, the invention starts out from the basic idea of taking into account varying variables (disturbance variables) which influence the coating thickness or bond coating thickness during coating, in particular during bonding, and of controlling bonding in accordance with their influence. During coating/bonding the coating thickness is measured and deviations from a desired value are readjusted. The temperature of the substrate(s) and the temperature of the bonding material, which influence the viscosity of the bonding material, are taken into account as varying variables. The influences of the disturbance variables on the coating thickness and bond coating thickness are determined empirically, and the aggregates involved in the coating and bonding process, such as a dosing pump, a dosing arm, a rotary drive for the coating material application and bonding material application as well as a connecting means for connecting the substrates and a rotary centrifugal drive are controlled in accordance with an algorithm which takes into account the influences of the disturbance variables, in order to achieve a coating thickness which meets a given desired value.
It is an advantage of the present invention that the coating thickness can be adjusted very accurately and that there is a very low reject rate, e.g. in connection with DVDs produced in accordance with the present invention, so that the production process leads to an increased yield.